This is invention relates to an improved process of volumetric reduction of what is retained in effluent treatment processes containing acetic derivatives submitted to a tangential filtration, through the use of surface-active agents chosen from among lignosulfonates, particularly those originating from processes for the obtainment of cellulose acetate.
The conventional process of cellulose acetate obtainment, known by those skilled in the art, contains simplified stages of: acetylation of the cellulose paste, saponification, concentration, precipitation, washing, pressing, and drying. The cellulose acetylation reaction usually occurs in an acetic environment and with an excess of the reagentxe2x80x94acetic anhydride, which is also hydrolysated to acetic acid. This solution of acetic acid, in addition to other obtained throughout the process, as for example, the solution that is obtained through the washing stage of the cellulose acetate, is what particularly relates to this invention. Or rather, the production of cellulose acetate results in the generation of an appreciable quantity of an aqueous solution containing 20 to 35% weight of acetic acid, which is absolutely necessary for recovering the acetic acid, of a high aggregated value, so as to make the production of cellulose acetate economically feasible.
This aqueous solution of 20 to 35% by weight of acetic acid obtained during the manufacturing of cellulose acetate contains organic impurities, mainly cellulose acetates and hemicelluloses, in suspension and in solution, which should be eliminated, in order to make the recovery of the acetic acid possible, as the presence of the impurities results in a foam or encrustation in the recovery columns, impairing the operation and increasing the frequency of stoppage for maintenance and cleaning.
It is important to stress the presence of the sulfuric acid that contaminates the acetic acid solution under study, as the same is used as a catalyst in the reactions of acetylation and hydrolysis. Usually, the concentration of sulfates, in the aqueous solution of acetic acid, 20 to 35% , resulting from the cleaning of the precipitated cellulose acetate, is on the order of 1000 ppm. The organic impurities and others in suspension are generally eliminated physically, through a separation process, which may be conventional filtration, obtaining a clear solution; and the organic impurities in solution are generally treated through warm hydrolysis, using the sulfuric acid to break the long sequences of hemicelluloses. By that treatment, the impurities continue to be present, but with molecular dimensions or weights that are much lower, in addition to requiring large retention timesxe2x80x94on the order of 15 to 24 hours, resulting in large dimensioned equipment. The temperatures required for the activation of the hydrolysis reaction are higher, on the order of 90xc2x0 C., which results in the higher consumption of steam, maintenance and operational problems.
The largest problem of this process is the increase in the concentration of sulfates in the acetic acid solution, that after hydrolysis, becomes 3500 ppm. This elevated degree of sulfates creates an environmental problem, as the water contained in the aqueous solution of acetic acid that constitutes the effluent of the recovery unit of the acetic acid, shall drag the sulfates contained after the hydrolysis through the environment, in a concentration of between 6000 and 7000 ppm. Another negative consequence of the elevated degree of sulfate to the exit from the hydrolysis is the chemical attack on the equipment of the acetic acid recovery unit, increasing the frequency of maintenance and consequently, its costs, requiring an attempt to diminish the frequency of maintenance, the use of high quality construction materials.
The hydrolysis is verified in the treated current, through a foam test, or rather, in the hydrolyzed solution, there should not be any foam. The absence of foam guarantees a controlled operation in the recovery columns of the acetic acid. However, the hydrolysis does not contribute to the reduction of maintenance problems or the plant shut downs, as the hydrolyzed impurities generate other impurities, that also becomes encrusted on the equipment, or rather, the impurities, continue to be present in the acetic acid solution; the acid hydrolysis simply transforms the original organic impurities, hemicelluloses and cellulose acetate in its majority, into other impurities, that due to the lower molecular weight, length of the sequence, or any other chemical characteristic, does not generate foam, allowing operations to be more controlled.